water forms, but the scallop is naturally more active, and is well 

 supplied with tactile tentacles as well as with eyes. 



Quick motions outside of an aquarium made so the illumi- 

 nation is not materially affected, and so the aquarium is not 

 jarred, frequently seem to cause response, but the results are 

 so frequently negative that apparent responses may be acci- 

 dental. Experiments to test the power of vision have not been 

 devised. 



Tentacles. — 



The number and arrangement of the tentacles has been dis- 

 cussed in treating the mantle. Although the size, shape, and 

 position of the tentacles differ considerably, they are all essen- 

 tially of the same structure. As they are included in the color 

 pattern of the margins of the mantle, some are pigmented and 

 others are not. The tentacles are quite smooth when they are 

 extended, and short, wrinkled, and conical when retracted. Each 

 tentacle is covered by a layer of epithelium and bears near its 

 free extremity several conical projections, "pinselzellen," each 

 of which bears a cluster of sense cilia at its tip. These projec- 

 tions are always more numerous near the extremities of the ten- 

 tacles than elsewhere, but they are scattered pretty well over 

 their surfaces and may occur on the mantle also. Each tentacle 

 (fig. 22) is supplied with a large nerve (n.) derived from the 

 circum-pallial nerve, that runs the whole length of the tentacle 

 near its middle line. Nerve cells are present in this nerve 

 throughout its length. A connective tissue framework divides 

 the interior into a number of spaces. Muscle fibers (mf.) that 

 run lengthwise of the tentacle lie alongside the framework 

 and surround the blood spaces (bs.). The nerve lies very near 

 the center of the framework and occupies one side of a large 

 blood space. The structure is very much the same as that of 

 the special sense tentacle of Yoldia (5). The chief difference 

 is, that in this tentacle there are a number of blood spaces, while 

 in Yoldia there is only one. 



The blood spaces serve to lengthen the tentacle, by having 

 blood forced into them, and the muscle fibers shorten it. 



Otocysts. — 



The otocysts (figs. 15 and 24, ot.) are placed very near, and 

 almost dorsal to the pedal ganglia, and accordingly not far from 



51 



